With ordinary light sources, light emitted by the source is usually of low optical coherence so that a fairly uniform light intensity profile is achieved at a target plane by superposing multiple wave fronts. In optical lithography or metrology an excimer laser is often used as a light source, and typically the radiation output from the laser is passed a radiation beam-scrambling illuminator to distribute the light intensity uniform over a particular area, e.g. an SLM area. Different laser sources have different coherence lengths. A problem is that coherent parts of a beam of radiation may sometimes cause an interference pattern on the target.